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Metamorphic Rocks “New Rock From Old” Metamorphic Rocks DVD Clip Metamorphic Rocks • Metamorphism – takes place when preexisting rock is subjected to temperature and pressures unlike those in which it formed. Metamorphic Rocks • Degrees of metamorphism (Composition) – Low-grade – the common sedimentary rock shale becomes more compact metamorphic rock slate – High-grade – features such as bedding planes, fossils, and vesicles that may have existed are obliterated. Formation of Metamorphic Rocks The diagram below shows how the type of rock that forms during metamorphism indicates the conditions under which the metamorphism occurred. Metamorphic Rocks • Metamorphic Settings – Contact metamorphism – when magma comes in contact with existing rock, heat changes the structure and mineral composition. – Regional metamorphism – changes in temperature & pressure over a large area result in changes in the texture, structure or chemical composition of a rock. Metamorphic Textures • Foliated Texture – the alignment of the minerals gives the rock a layered or banded appearance. (parallel alignment or perpendicular to the compression) • Foliated Rocks – Slate – very-fine grained and composed of mica flakes. Excellent cleavage. (most useful for roof and floor tiles, chalkboards, and billiard tables) – Schists – coarse-grained rock that forms when large amounts of heat and pressure are exerted on slate. – Gneiss – banded metamorphic rocks that contain mostly elongated and granular minerals. (most common minerals are quartz & feldspar) Metamorphic Textures • Non-foliated Texture – resembles a coarse- grained igneous rock. (marble & quartzite) • Non-foliated Rocks – Marble – a coarse, crystalline rock whose parent rock is limestone. • Has a hardness of 3 on Mohs Hardness Scale, so it is a popular building stone and comes in a variety of colors. – Quartzite – a very hard metamorphic rock whose parent rock is quartz sandstone.
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